Oscillation suppression

ABSTRACT

The invention relates to oscillation suppression and, more particularly, concerns a method and apparatus for suppressing oscillation in a signal identified as or suspected of containing an oscillation due to feedback.  
     The method involves converting the signal into frequency bands in the frequency domain, applying, for a selected period of time, a randomly changing phase to the signal in at least one of said frequency bands, and reconverting the converted signal into an output waveform signal. The selected period is divided into a series of successive time windows, and for each successive time window a newly generated random or pseudo-random phase is applied to the signal. The method can be used in combination with a method for detecting oscillation in said signal in each of the frequency bands, a randomly changing phase applied in each frequency band for which said oscillation has been detected.  
     The invention has particular application in hearing aid devices.

FIELD OF THE INVENTION

[0001] The present invention relates to oscillation suppression and,more particularly, concerns a method and apparatus for suppressingoscillation in a signal identified as or suspected of containing anoscillation due to feedback. The present invention may be used inconjunction with the method and apparatus for identifying oscillation ina signal due to feedback described in applicant's copending applicationentitled ‘Oscillation Detection’ (Attorney ref. 30-517-4681).

BACKGROUND OF THE INVENTION

[0002] In this specification, where a document, act or item of knowledgeis referred to or discussed, this reference or discussion is not anadmission that the document, act or item of knowledge or any combinationthereof was at the priority date:

[0003] (i) part of common general knowledge; or

[0004] (ii) known to be relevant to an attempt to solve any problem withwhich this specification is concerned.

[0005] Acoustic amplifiers are used in many common applications such astelephones, radios, headsets, hearing aids, and public address systems.Typically, such an application comprises a microphone or other inputtransducer to pick up sounds and convert them into an electrical signal,an electronic amplifier to increase the power of the electrical signal,and a speaker or other output transducer to convert the amplifiedelectrical signal back into sound.

[0006] If the input and output transducers are close enough, the outputacoustic signal may be picked up by the input transducer and fed backinto the amplifier with a delay, the delay being the time taken for thesound to travel from the output transducer to the input transducer (plusany delay due to the electrical processing of the signal). This is‘acoustic feedback’. Electrical feedback can also occur if theelectrical signal at the output is coupled back to the input, forexample by inductive or capacitive coupling. Further, mechanicalfeedback can also occur if vibrations are transmitted from the outputtransducer to the input transducer via the body or case of theamplification system. Under feedback conditions, the device can thenbecome unstable and the components begin to ring. The ringing thenself-reinforces and increases in intensity to drive the components intosaturation. FIG. 1 illustrates a feedback loop, showing diagrammaticallythe components in an acoustic amplifier circuit, namely microphone 1,amplifier 2 and speaker 3, with feedback loop 4 representing the outputsignal feeding back to the input transducer.

[0007] All forms of feedback may result in instability or oscillation ofthe output signal from the amplifier under certain conditions.Oscillation and instability are undesirable because they distort thesignals being amplified and can result in very loud unpleasant sounds.In the case of hearing aids, this can lead to problems both for thewearer and for those around. The conditions for oscillation are that thetotal gain around the loop must be greater than 1, so that the signal isfed back into the system with a greater intensity each time, and thetotal delay around the loop must be a whole number of periods of theoscillation frequency, so that the input and output signals addconstructively. Equivalently, the total phase change around the loopmust be a multiple of 2π radians for the oscillation frequency. Thesecriteria are set out in equations 1 to 3 below.

Loop Gain>1  (eq. 1)

Loop Delay=N×period  (eq. 2)

Loop Phase Change=2Nπ radians  (eq. 3)

[0008] (where N is a positive integer)

[0009] Any electronic system containing a microphone and speaker inclose proximity may suffer from acoustic feedback. In hearing aids, thisoften results in the wearer experiencing unpleasant audible effects suchas loud whistling tones at certain frequencies, usually highfrequencies.

[0010] The traditional procedure for increasing the stability of ahearing aid is to reduce the gain at high frequencies, as suggested in,for example, U.S. Pat. No. 4,689,818. This may be done by setting themaximum gain value for each frequency, or automatic high frequency (HF)gain roll-off may be used. Controlling feedback by modifying the systemfrequency response, however, means that the desired high-frequencyresponse of the instrument must be sacrificed in order to maintainstability.

[0011] Efforts have been undertaken to reduce the susceptibility ofhearing aids to feedback oscillation by improving the fit and insulatingproperties of the ear mould. Efforts have also been undertaken from anelectrical standpoint, from attenuation and notch filtering, asdisclosed in U.S. Pat. No. 4,088,835, to estimation and subtraction ofthe feedback signal, as disclosed in U.S. Pat. No. 5,016,280, tofrequency shifting or delaying the signal, as disclosed in U.S. Pat. No.5,091,952. Many different approaches to an electrical solution withcontinuous monitoring of the feedback path have been documented in therelevant literature.

[0012] A technique commonly used to suppress feedback in public addresssystems is a frequency shift, in which the input signal is altered by afew Hertz prior to being output at the receiver. This approach has notbeen particularly successful in hearing aids because a large frequencyshift is required to achieve a significant increase in gain. In hearingaids, the distance between microphone and receiver is much smaller thanin public address systems, and thus a feedback signal with only a smallfrequency shift may still be relatively closely in phase with the input.

[0013] Signal phase can also be altered by using a time-varying delay[1]. While this can provide 1-2 dB of additional useable gain, it canalso result in an audible ‘warbling’ effect. All pass filters have alsobeen used to modify the phase response of the feedback loop, but it canbe difficult to achieve satisfactory phase at all frequencies. Methodshave been proposed to push danger regions in the phase response tofrequencies outside the primary audio range where suppression can beapplied without loss of sound quality [2] [3]. These techniques stillassume that the feedback path is constant however, and no suggestion hasbeen made that an adaptive implementation may be developed.

[0014] The most common gain altering approaches attempt to reduce thesystem gain only in narrow bands where feedback is likely to occur. Thishas been attempted with a variety of notch filter implementations [1][4] [5]. Adaptive notch filtering has allowed 3-5 dB of additionaluseable gain. Two of the biggest problems with notch filteringtechniques have been the inability to accurately track the variations inthe feedback path with a narrow band, and the effects on normal spectralcontent with a broader band. In addition, the notch filter can actuallycontribute an additional phase change to the loop and shift thefrequency of oscillation as soon as it is applied.

[0015] Substantial increases in useable gain have been achieved byinserting an additional feedback path, based on an estimation of thereal feedback path, but 180 degrees out of phase. Early adaptiveimplementations of such systems performed continuous estimation of thefeedback path by inserting noise signals with appropriate statisticalproperties at the receiver and correlating the output with the input atthe microphone [1] [6]. These reported up to 10 dB of additional useablegain [7] but, since the noise ‘test’ signals were audible and unpleasantfor most wearers, this particular technique never became particularlywidespread.

[0016] More recent feedback cancellation systems of this type rely onsounds in the environment to perform their correlation [8]. To avoidartefacts and incorrect suppression of speech however, the estimationtime has to be longer than in systems using unnatural sounds to performcorrelation. This means that sudden changes in the feedback path canresult in several seconds of whistling before successful cancellationoccurs. If implemented in conjunction with another technique to handlesudden changes, this approach can allow at least 10 dB of additionaluseable gain [9]. The benefits and limitations of such systems arediscussed in [10].

[0017] Nearly all of the techniques discussed here require someknowledge of the frequency of oscillation. However, as a result of thenature of direct and multiple reflected acoustical paths betweenmicrophone and speaker (or the changing acoustic properties of theear/earmould/hearing aid coupling with regard to hearing aids) thefrequency of acoustic feedback is unpredictable and may extend over asubstantial portion of the audio frequency spectrum (between 20 and20,000 Hz). As a result, it is desirable to have a circuit that canquickly identify an oscillation and its frequency.

[0018] U.S. Pat. Nos. 4,232,192 and 4,079,199 propose systems using aphase locked loop (PLL) adapted to recognize an oscillation when itoccurs. As is known, however, when the input signal falls off, a PLLtends to become unstable and to drift. The result of the drift is anundesirable periodic, acoustic noise signal.

[0019] U.S. Pat. No. 4,845,757 describes another oscillation recognitioncircuit. This circuit detects oscillations by looking for long-lastingalternating voltages having relatively large amplitude and relativelyhigh frequency. This is problematic in many applications because itmeans that the signal may contain feedback oscillations for some timebefore they are identified by such a circuit.

[0020] There remains a need in the art to provide an improved or atleast an alternative way of detecting oscillations in a signal in areliable, effective and rapid manner, and to apply appropriatesuppression to the signal upon detection.

SUMMARY OF THE INVENTION

[0021] The invention provides, in accordance with a first aspect, amethod for suppressing oscillation in a signal identified as orsuspected of containing an oscillation, the method comprising thefollowing steps:

[0022] converting the signal into frequency bands in the frequencydomain;

[0023] applying, for a selected period of time, a randomly changingphase to the signal in at least one of said frequency bands; and

[0024] reconverting the converted signal into an output waveform signal.

[0025] This method has the effect of disrupting the consistentconstructive addition of the feedback signal to the input signal,providing a simple but very effective solution to the suppressionproblem.

[0026] Preferably, said selected period is divided into a series ofsuccessive time windows, and for each successive time window a newlygenerated random or pseudo-random phase is applied to the signal. Thistechnique thus provides the randomly changing signal phase.

[0027] The method may be applied in combination with a method fordetecting oscillation due to feedback in said signal in each of saidfrequency bands, a randomly changing phase applied in each frequencyband for which said oscillation has been detected.

[0028] The oscillation detection technique may comprise calculating, foreach frequency band, the change in signal phase and/or signal amplitudefrom a time window to a subsequent time window, and comparing, for someor all of said frequency bands, the results of the calculation step todefined criteria to provide a measure of whether oscillation due tofeedback is present in the signal.

[0029] Alternatively, the oscillation detection technique may be a phaselocked loop method, or may involve detection of a large sustainedamplitude in a particular frequency band.

[0030] The randomly changing phase may be applied in each frequency bandto a gain value to be applied to the signal.

[0031] In a preferred form, the method includes the step of, for aparticular frequency band, generating a complex number with random orpseudo-random phase and amplitude 1.0 for each successive time window,and applying this complex number to the signal in that frequency band. Areal gain value for said frequency band may be multiplied by saidcomplex number before the gain is applied to the signal.

[0032] In an alternative form, the method may include the step of, for aparticular frequency band and in each successive time window, replacingthe signal or signal gain with a signal or signal gain having equalamplitude and a random or pseudo-random phase.

[0033] The invention provides, in accordance with a second aspect, anapparatus for suppressing oscillations in a signal identified as orsuspected of containing an oscillation, comprising:

[0034] means for converting the signal into frequency bands in thefrequency domain;

[0035] means for applying, for a selected period of time, a randomlychanging phase to the signal in at least one of said frequency bands;and

[0036] means for reconverting the converted signal into an outputwaveform signal.

[0037] The apparatus preferably includes means for dividing the signalinto a series of successive time windows, and means for applying to thesignal, for each successive time window, a newly generated random orpseudo-random phase.

[0038] Preferably, the apparatus is provided in combination with a meansfor detecting oscillation due to feedback in said signal in each of saidfrequency bands, the means for applying arranged to apply a random phasein each frequency band for which said oscillation has been detected.

[0039] The means for detecting oscillation may comprise means forcalculating, for each frequency band, the change in signal phase and/orsignal amplitude from a time window to the next, and means forcomparing, for some or all of said frequency bands, the results of thecalculation step to defined criteria to provide a measure of whetheroscillation due to feedback is present in the signal. Alternatively, themeans for oscillation detection may comprise phase locked loopcircuitry, or means for detection of a large sustained amplitude in aparticular frequency band.

[0040] In a preferred form, the means for applying are arranged to applythe randomly changing phase in each frequency band to a gain value to beapplied to the signal.

[0041] The apparatus may include means for generating a complex numberwith random or pseudo-random phase and amplitude 1.0 for each successivetime window, and means for applying this complex number to the signal inthat frequency band.

[0042] Preferably, means are included for multiplying a real gain valuefor said frequency band by said complex number before applying the gainto the signal.

[0043] In an alternative form, the apparatus includes means for, for aparticular frequency band and in each successive time window, replacingthe signal or signal gain with a signal or signal gain having a randomor pseudo-random phase.

[0044] The invention provides alteration of the feedback loop in amanner that disrupts the feedback oscillation conditions and suppressesthe oscillation without significantly affecting the system frequencyresponse. If used with an appropriate oscillation detection technique,oscillation can be detected and suppressed very rapidly, and beforeaudible ringing results.

[0045] The randomly changing phase is added in successive time windowsover a certain length of time, for example approximately one second, toany frequency that appears to be in a state of oscillation. The lengthof time may be preselected, or may be dynamically determined withreference to the result of oscillation detection in that frequency band.The random phase variation suppresses the oscillation by disrupting theconsistent constructive addition of the feedback signal to the inputsignal.

[0046] It should be noted that the feedback suppression method of theinvention may be used with any suitable feedback detection approach.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The present invention will become more apparent by describing indetail a preferred non limiting embodiment with reference to theattached drawings, in which:

[0048]FIG. 1 is a block diagram schematically illustrating a feedbackloop;

[0049]FIG. 2 is a block diagram of an apparatus according to the presentinvention;

[0050]FIG. 3 is a flow diagram illustrating the logic and process offeedback detection;

[0051]FIG. 4 is a flow diagram illustrating the logic and process offeedback suppression; and

[0052]FIGS. 5 and 6 are block diagrams of alternative architectures ofapparatus according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0053] An acoustic system 10 in accordance with the invention, such as ahearing aid, is schematically depicted in FIG. 2. A microphone 11converts an acoustic signal, such as the speech, into an analogueelectrical signal proportional to the acoustic signal, which signal isthen converted by an A/D converter 12 into a digital signal. The outputof A/D converter 12 is connected to the input of a Discrete FourierTransform (DFT) unit—such as a Fast Fourier Transform (FFT) unit 13—foranalysing the frequency components of the signal, and unit 14 enablesanalysis of 64 frequency bands across the spectrum of the signal. Asuitable unit is the Toccata Plus integrated circuit designed anddeveloped by the Dspfactory, operating with 16 kHz sampling rate andusing 128 point windows of 8 millisecond duration with 50% overlap toyield 64 linearly spaced frequency bands at 125 Hz intervals from 0 to8000 Hz. Module 20 is a feedback detector arranged to monitor the phaseand amplitude of the signal in each frequency band in the spectrum(adjusted if appropriate, as explained further below) during successivesampling windows at short intervals, such as successive 8 millisecondwindows with 50% overlap, calculated every 4 milliseconds. The apparatusincludes a counter for each frequency band, which can be incremented orreset at each successive time window.

[0054] For each time window, the measured phase from the previous windowis subtracted from the phase in the current window to calculate thechange in phase at a particular frequency band. This change in phase iscompared to the previous change in phase. If the values are within adefined variation (ie the change in the phase change is within thethreshold) then the counter is incremented, otherwise the counter isreset. Further, the amplitude in the current window is compared with theamplitude in the previous window. If the current amplitude is less thanthe previous amplitude, then the counter is reset. The feedback detectoris programmed to respond—by triggering feedback suppression—to thecounter reaching a value M. The present invention contemplates thateither the change in phase change criterion (counter reaches M_(p)) orthe change in amplitude criterion (counter reaches M_(a)) may beconsidered for suppression triggering, or both.

[0055] The example represented in FIG. 3 illustrates, for a time window,the process of detection using the change in phase change criterion. Foreach of the 64 bands, the state of the band is determined (30). If thatband is already being suppressed (31), no calculations are performed.Otherwise, the phase is calculated (32), and the previous phase valuecalculated for that band (which value has been stored—see below) issubtracted from the current phase value (33) to provide a current valueof phase change. The next step (34) is to subtract the previous phasechange value from the current phase change value, to output a value ofchange of phase change. This value is then checked (35) and (37), and ifit is within a prescribed threshold for phase change variation, thecounter is incremented by 1 (41). The subtraction of 2π (36) and secondcheck (37) ensure that the value of the change of phase change ischecked, irrespective of whether the change has increased or decreased.If the value is not within the threshold, the counter is reset to 0(38), the current phase and phase change value is saved (39), and thenext band is selected (40).

[0056] If the counter has been incremented (41), a check is made todetermine if it has reached a value M_(p)

[0057] (42), thereby indicating an oscillation has been detected (43)and flagging that band for suppression (see below). If not, the currentphase and phase change values are saved (39), and the next band isselected (40). It is to be noted that the bands can be checked inparallel or sequentially within each time window.

[0058] In simulations carried out by the inventors, where both criteriafor detection have been employed, M_(a)=M_(p)=12 gives good performance.Using M_(a)=M_(p) simplifies the detection apparatus and method, as acommon counter can be used. If only one criterion is to be employed indetecting feedback, the M_(a) or M_(p) value may be increased to avoidfalse triggering of feedback suppression.

[0059] Once the counter for any frequency band exceeds the requiredvalues of M_(a) and/or M_(p), this frequency band is deemed to be inoscillation, and an apply phase module 21 is triggered (see FIG. 2).Apply phase module 21 generates a complex number with random phase andamplitude 1 for each window, and multiplies the real gain value atmodule 22 for the frequency band by this complex number before the gainis applied to the signal via gain unit 23 to provide an adjustedspectrum 24. The loop illustrated in FIG. 2 indicates that the phase ofthe gain multipliers depends on the apply phase unit, which depends onthe feedback detector unit. Apply phase module 21 continues to applyrandom phase to the gain for about one second, to allow the conditionswhich created the feedback path to change.

[0060] The example represented in FIG. 4 illustrates the process ofsuppression for a time window, appropriate for the example embodimentsillustrated in FIGS. 5 and 6. Firstly, the state of a selected band ischecked (40), to determine whether it is flagged for suppression (41).If not, the next band is selected (47). If it is flagged forsuppression, the magnitude of the signal at that band is obtained (42)and multiplied by the real part of the generated random complex number(43), the resulting new real component being saved (44). Further, themagnitude of the signal is multiplied by the corresponding imaginarypart of the generated random complex number (45), and the resulting newimaginary component saved (46).

[0061] The signal passes through MPO unit (Maximum Power Output) 25 (seeFIG. 2), and is then reconverted into a time domain waveform by inverseFFT module 26. A D/A converter 27 then converts the digital signal to anelectrical analogue signal before supplying it to the hearing aid outputterminal to drive speaker 28.

[0062] It is to be noted that the ‘magnitude of the signal’ in a bandreferred to above in the context of FIG. 4 may be the output spectrumvalue (for the example embodiments shown in FIGS. 5 and 6), or may bethe gain value (for the example embodiment shown in FIG. 2), and theinvention may be implemented using either approach, the selectiondepending at least in part on the hardware employed for the processing.In the alternative architectures of FIGS. 5 and 6 the random phase isapplied to the output spectrum rather than to the gains, in bothembodiments the gain values are applied to the signal by gain unit 23before feedback detector 20. In FIG. 6, MPO unit 25 is omitted, toillustrate that the invention can be implemented without it.

[0063] Feedback detector 20 and apply phase module 21 do not necessarilyhave to be applied together. An alternative form of feedbacksuppression, such as application of a notch filter, may be applied to asignal in which feedback oscillation has been identified by feedbackdetector 20. Other types of feedback suppression which might be employedinclude gain attenuation at the frequency band in question, applying atime varying phase change, or subtraction of the signal at the frequencyband in question. Similarly, an alternative form of feedback detector,such as a phase locked loop (PLL) circuit, may be employed, apply phasemodule 21 being used to apply a random phase to the signal in thatparticular frequency band once feedback has been detected.

[0064] It has been found in simulations carried out by the inventorsthat application of both feedback detector 20, combining the monitoringof both phase change and amplitude, along with the application of applyphase module 21, can result in suppression of all feedback oscillationin 60-100 milliseconds.

[0065] Modifications and improvements to the invention will be readilyapparent to those skilled in the art. Such modifications andimprovements are intended to be within the scope of this invention. Forexample, in accordance with the invention, the signal spectrum may besplit into a plurality of discrete frequency bands, or alternativelyneighbouring bands may overlap.

[0066] The word ‘comprising’ and forms of the word ‘comprising’ as usedin this description and in the claims does not limit the inventionclaimed to exclude any variants or additions

1. A method for suppressing oscillation in a signal identified as orsuspected of containing an oscillation, the method comprising thefollowing steps: converting the signal into frequency bands in thefrequency domain; applying, for a selected period of time, a randomlychanging phase to the signal in at least one of said frequency bands;and reconverting the converted signal into an output waveform signal. 2.The method of claim 1, wherein said selected period is divided into aseries of successive time windows, and for each successive time window anewly generated random or pseudo-random phase is applied to the signal.3. The method of claim 1, further comprising the steps of detectingoscillation due to feedback in said signal in each of said frequencybands, and of applying a randomly changing phase in each frequency bandfor which said oscillation has been detected.
 4. The method of claim 3,wherein the randomly changing phase is applied in each frequency band toa gain value to be applied to the signal.
 5. The method of claim 3, inwhich the oscillation detection technique comprises calculating, foreach frequency band, the change in signal phase and/or signal amplitudefrom a time window to a subsequent time window, and comparing, for someor all of said frequency bands, the results of the calculation step todefined criteria to provide a measure of whether oscillation due tofeedback is present in the signal.
 6. The method of claims 3, in whichthe oscillation detection technique is a phase locked loop method. 7.The method of claim 3, in which the oscillation detection techniqueincludes detection of a large sustained amplitude in a particularfrequency band.
 8. The method of claim 2, including the step of, for aparticular frequency band, generating a complex number with random orpseudo-random phase and amplitude 1.0 for each successive time window,and applying this complex number to the signal in that frequency band.9. The method of claim 8, in which a real gain value for said frequencyband is multiplied by said complex number before the gain is applied tothe signal.
 10. The method of claim 2, including the step of, for aparticular frequency band and in each successive time window, replacingthe signal or signal gain with a signal or signal gain having equalamplitude and a random or pseudo-random phase.
 11. An apparatus forsuppressing oscillations in a signal identified as or suspected ofcontaining an oscillation, comprising: means for converting the signalinto frequency bands in the frequency domain; means for applying, for aselected period of time, a randomly changing phase to the signal in atleast one of said frequency bands; and means for reconverting theconverted signal into an output waveform signal.
 12. The apparatus ofclaim 1 including means for dividing the signal into a series ofsuccessive time windows, and means for applying to the signal, for eachsuccessive time window, a newly generated random or pseudo-random phase.13. The apparatus of claim 11, further comprising a means for detectingoscillation due to feedback in said signal in each of said frequencybands, and wherein the means for applying are arranged to apply a randomphase in each frequency band for which said oscillation has beendetected.
 14. The apparatus of claim 13, in which the means fordetecting oscillation comprises means for calculating, for eachfrequency band, the change in signal phase and/or signal amplitude froma time window to the next, and further comprising means for comparing,for some or all of said frequency bands, the results of the calculationstep to defined criteria to provide a measure of whether oscillation dueto feedback is present in the signal.
 15. The apparatus of claim 11,wherein the means for applying are arranged to apply the randomlychanging phase in each frequency band to a gain value to be applied tothe signal.
 16. The apparatus of claim 13, in which the means foroscillation detection comprises phase locked loop circuitry.
 17. Theapparatus of claim 13, in which the means for oscillation detectioncomprises means for detection of a large sustained amplitude in aparticular frequency band.
 18. The apparatus of claim 12, includingmeans for generating a complex number with random or pseudo-random phaseand amplitude 1.0 for each successive time window, and means forapplying this complex number to the signal in that frequency band. 19.The apparatus of claim 18, including means for multiplying a real gainvalue for said frequency band by said complex number before applying thegain to the signal.
 20. The apparatus of claim 12, including means for,for a particular frequency band and in each successive time window,replacing the signal or signal gain with a signal or signal gain havinga random or pseudo-random phase.